The present invention relates generally to a membrane exchanger and more particularly to a hollow fiber exchanger which avoids shunting and is easy to manufacture.
During certain surgeries, natural cardiovascular activity is suspended. Accordingly, the bloodstream requires extracorporeal oxygenation in lieu of the lungs. Exchangers perform this oxygenation of the blood through use of one or more membranes which each allow gasses to pass freely, but resist liquids from passing across each membrane. In this way, oxygen is added to and carbon dioxide is removed from the bloodstream.
Exchangers come in many shapes and forms, but generally, there is a liquid or a blood conduit and a gas or an air conduit. In membrane exchangers, the liquid does not typically contact the gases directly. The air and blood conduits are divided by membranes in an exchange chamber. At an inlet end, the air conduit is divided to flow within a number of hollow fiber membranes. At the inlet end of the hollow fibers, the air is rich in oxygen, however, at an outlet end, the air is oxygen depleted and carbon dioxide rich. The blood conduit starts at an inlet port, travels outside of the hollow fiber membranes within the exchange chamber and exits an outlet port. Potting around the ends of the hollow fiber membranes keeps the blood from escaping the exchanger or directly mixing with the air.
To avoid shunting, the hollow fibers should be tightly packed within the exchange chamber. The term xe2x80x9cshuntingxe2x80x9d describes an undesired process whereby the blood avoids contact with the hollow fibers such that oxygenation is stifled. For example, when the hollow fibers are not tightly packed against the walls of the chamber, blood can flow within the resulting gap while passing from the inlet port to the outlet port, therefore bypassing the fibers.
Conventional exchangers attempt to solve the shunting problem in a variety of ways. In one example, the size of the fiber bundle is tightly controlled so that when the bundle is inserted into an enclosure, the fibers are tightly packed against the wall. The insertion involves sliding the fiber bundle into the enclosure. Unfortunately, tight control of the fiber bundle size is difficult such that shunting occurs if the bundle is too small. On the other hand, if the bundle is too large, the bundle will not slide into the enclosure easily and can result in damaging the fibers. These problems have plagued prior attempts to manufacture exchangers which solve the shunting problem. Further, these exchangers are generally difficult and/or expensive to manufacture.
Potting material is used during manufacture of the exchanger to seal the inlet and outlet ends of the hollow fibers from the blood within the exchange chamber. Typically, during manufacture an external reservoir is connected to the exchanger. A tube connected to the reservoir allows fluid communication with an area near the ends of the hollow tubes. Either gravity or centrifugal force is used to cause uncured potting material to flow and hold it in place while curing. Unfortunately, the potting reservoir and tube generally becomes fouled with potting material after a few uses and must be replaced. Additionally, when subjected to the centrifugal force, the tube can come loose which results in potting material being sprayed out of the reservoir.
Methods using centrifugal force generally pot one end of the exchanger at a time. The exchanger is first spun about a first end to create centrifugal force which disburses the potting material from the potting reservoir to a second end of the exchanger. After curing the second end, the exchanger is spun about the second end to disburse potting material from another potting reservoir to a first end of the exchanger. In this way, both ends of the exchanger are successively potted.
After curing, any excess potting material is trimmed away. During trimming a portion of the hollow tube ends are typically removed along with the excess potting material. A sharp tool is used to slice away the unwanted portion of potting material. Unfortunately, the potting material can delaminate from the wall of the exchange chamber during the trimming process. When the sharp tool is used to trim, the blade temporarily compresses the potting material such that it may peel away from the wall. Even though the potting material may resume its uncompressed shape after delamination, during use, the blood and air can pass along the wall and around the potting material. As can be appreciated, delamination ruins the exchanger.
A need, therefore, exists for an exchanger design which avoids shunting without being difficult to manufacture. During manufacture, such an exchanger should eliminate the need for external potting reservoirs. Moreover, such an exchanger should not be susceptible to delamination during the trimming process.
In accordance with the present invention, an exchanger and method for manufacturing the same are disclosed. In one embodiment, the exchanger has a cylindrical case which crimps a hollow fiber bundle along a length of the cylindrical case in at least two radial positions. At least one crimp is between an inlet port and an outlet port of a blood conduit. Preferably, the cylindrical case has two portions with different radii so that when the two portions are mated to enclose the hollow fiber bundle, crimping is achieved. In this way, shunting of blood is avoided by being forced to pass a crimped position between the inlet and outlet ports to more evenly distribute the blood among the fibers of the bundle.
In another embodiment, a method for making an exchanger includes filling a chamber, integral to the exchanger, with uncured potting material. The exchanger is spun to distribute the uncured potting material through dispensing holes in the chamber ends and about both open ends of an encasement. After curing, a portion of the potting material and the hollow fiber bundle are trimmed away. Preferably, the cured potting material is molded such that the trimming is on a surface of the potting material away from an annular relief to help avoid delamination.
Based upon the foregoing summary, a number of important advantages of the present invention are readily discerned. Crimping the hollow fiber bundle in the blood conduit avoids shunting. Further, a potting reservoir integral to the exchanger avoids the problems associated with external potting reservoirs. Further still, delamination during trimming is avoided by the addition of the annular relief.